Functions are one of the most important parts of JavaScript. They allow you to write a block of code once and reuse it many times. This makes your code cleaner, more organized, and easier to maintain.

Function Syntax

function functionName(param1, param2, ...) {
  // code to execute
}

You define a function using the function keyword, followed by the function name, and parentheses. Inside the curly braces {}, you write the code that should run when the function is called.

Defining a Function

function greet() {
  alert("Hello World!");
}

This greet function shows a simple alert box. It does not take any parameters.

Calling a Function

greet();

When you call a function, JavaScript runs the code inside it.

Function Parameters

You can pass values to a function using parameters.

function greet(firstName, lastName) {
  alert("Hello " + firstName + " " + lastName);
}

greet("Steve", "Jobs");

JavaScript functions are flexible — they accept different types of parameters:

greet("Bill", "Gates");    // Hello Bill Gates
greet(100, 200);           // Hello 100 200

Passing Extra or Fewer Arguments

greet("Steve", "Jobs", "Mr."); // Hello Steve Jobs
greet("Bill");                 // Hello Bill undefined
greet();                       // Hello undefined undefined

Extra arguments are ignored, and missing ones become undefined.

Using arguments Object

Inside a function, you can use the built-in arguments object to access all passed values, even if you didn’t define parameters.

Returning a Value

You can return a result using the return keyword.

function getNumber() {
  return 10;
}

let result = getNumber();
console.log(result); // 10

Example: Return Sum

function sum(num1, num2) {
  return num1 + num2;
}

let result = sum(10, 20); // 30

Returning Another Function

function multiply(x) {
  function inner(y) {
    return x * y;
  }
  return inner;
}

let triple = multiply(3);

console.log(triple(2)); // 6
console.log(triple(3)); // 9

Function Expression

You can assign a function to a variable. This is called a function expression.

let add = function(num1, num2) {
  return num1 + num2;
};

console.log(add(10, 20)); // 30

Anonymous Function

An anonymous function has no name. It's often used as an argument or inside array methods like map().

let numbers = [10, 20, 30, 40];

let squares = numbers.map(function(number) {
  return number * number;
});

console.log(squares); // [100, 400, 900, 1600]

Arrow Function

Arrow functions are shorter versions of anonymous functions. They do not have their own this.

let square = num => num * num;

console.log(square(5)); // 25

Nested Functions

A function can be defined inside another function. Inner functions can use variables from the outer function.

function greet(firstName) {
  function sayHello() {
    alert("Hello " + firstName);
  }

  return sayHello();
}

greet("Steve");